#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cmath>
using namespace std;
const int maxn=15;

int n,a[maxn];

void input()
{
	for(int i=0;i<n;i++)  cin>>a[i];
	sort(a,a+n);
}

bool judge(int x,int y,int z)
{
	if(x+y>z && x+z>y && y+z>x)  return true;
	return false;
}

double Area(int x,int y,int z)
{
	double xx=x,yy=y,zz=z;
	double p=(xx+yy+zz)/2.0;
	return sqrt(p*(p-xx)*(p-yy)*(p-zz));
}

void solve()
{
	double ans=0.0;
	for(int i=n-1;i>=2;)
	{
		int x=a[i],y=a[i-1],z=a[i-2];
		if(judge(x,y,z))
		{
			ans+=Area(x,y,z);
			i-=3;
		}
		else i--;
	}
	printf("%.2lf\n",ans);
}

int main()
{
	while(scanf("%d",&n)!=EOF)
	{
		if(n==0)  break;
		input();
		solve();
	}
}

